The present invention relates to an ocular lens for binoculars, astronomical telescopes and the like, which may be made in low cost with a wide viewing angle.
The following three-element types of ocular lens systems for binoculars or the like are known;
i) A three-lens element system for an ocular lens for binoculars (for example, Japanese Patent Unexamined Publication No. 51-120231);
ii) A three-unit and three-element system for an ocular lens for viewfinders, which uses a plastic material as an optical material with an aspherical surface (for example, Japanese Patent Unexamined Publication Nos. 60-227215 and 63-318514); and
iii) A two-unit and three-element system for a viewfinder ocular system, which uses only optical glass for lens elements (for example, Japanese Patent Unexamined Publication No. 61-48810).
However, the foregoing systems suffer from the following disadvantages:
The ocular system i) suffers from a disadvantage that a curvature of field, an astigmatism and the like could not sufficiently be compensated for in a wide viewing angle. The ocular system ii) suffers from a disadvantage that it would be difficult to compensate for aberrations in a wide viewing angle due to the generation of the transversal chromatic aberration and the like although the curvature of field would be small. Also, the ocular system ii) and iii) have a small viewing angle. It should be also noted that any of the systems i) to iii) could not be manufactured at a low cost.
On the other hand, there have been proposed ocular systems each of which includes four to six lens elements in order to carry out a good aberration compensation covering a marginal portion of the view field (see Japanese Patent Examined Publication Nos. 63-6851 and 60-48014).
However, in the types having four or more lens elements, there is a serious problem that the manufacturing cost therefor becomes increased and a physical size would be increased. In order to perform a satisfactory aberration compensation with the ocular system which consists of four lens elements, it is necessary that an optical glass having a high refractive index be used for each lens and a radius of curvature be shortened. As a result, it is difficult to carry out the machining so that the manufacturing cost is increased. In particular, it is very difficult to make an aspherical surface on a glass lens, resulting in increase of manufacturing cost. In order to cope with these problems, it is proposed to use a plastic lens element. However, in general, the refractive index of the plastic is low so that it would be difficult to fully compensate for the aberration, particularly, coma aberration and spherical aberration.